Steel sheet having excellent rust resistance

ABSTRACT

A STEEL HAVING EXCELLENT RUST RESISTANCE COMPRISING 0.001 TO 0.15% OF CARBON, 0.01 TO 1.0% OF SILICON, 0.01 TO 1.0% OF MANGANESE 0.01 TO 0.5% OF COPPER. 0.003 TO 0.3% OF SULFUR AND TITANIUM IN AN AMOUNT ENOUGHT TO ATTAIN A TI/S RATIO OR 2 OR MORE, WITH THE BALANCE BEING IRON AND UNAVOIDABLE IMPURITIES.

United States Patent 3,832,166 STEEL SHEET HAVING EXCELLENT RUST RESISTANCE Hideya Okada, Yokohama, Haruo Shimada, and Kazuo Yamamoto, Tokyo, Japan, assignors to Nippon Steel Corporation, Tokyo, Japan No Drawing. Filed Nov. 30, 1972, Ser. No. 310,846 Claims priority, application Japan, Dec. 4, 1971, 46/ 97,572 Int. Cl. C22c 39/54 US. Cl. 75-125 1 Claim ABSTRACT OF THE DISCLOSURE A steel having excellent rust resistance comprising 0.001 to 0.15% of carbon, 0.01 to 1.0% of silicon, 0.01 to 1.0% of manganese, 0.01 to 0.5% of copper, 0.003 to 0.3% of sulfur and titanium in an amount enough to attain a Ti/S ratio of 2 or more, with the balance being iron and unavoidable impurities.

The present invention relates to steel sheet having excellent rust resistance, useful as materials for automobile and can-making.

Hitherto now, it has been often experienced that the steel sheet above-mentioned are quite susceptible to rust formation during their production process or during their shipment to users, and studies for clarifying the causes for rust formation and their prevention have been urged.

The present inventors have made extensive studies for clarifying the causes of rust formation, and have discovered that the rust formation is attributed directly to the fact that u(Mn-Fe)S contained in the steel is dissolved into dews formed on the steel surface. Namely it has been found that this m(Mn-Fe)S changes its conditions under both of an oxidizing atmosphere and a reducing atmosphere and causes the rust formation. Therefore, it is necessary to make the sulfur content in the steel as low as possible so as to lower the a(MI1'F6)S content in the steel in order to prevent or suppress the rust formation. However, it requires high cost to reduce the sulfur content in the steel, and yet it is impossible to completely remove the sulfur content. Therefore, other methods must be relied upon to reduce or prevent the formation of the Based on the above discoveries, the present inventors have developed steel sheets having excellent rust resistance by fixing the sulfur content as sulfide unsolubles into the dews.

One of the objects of the present invention is to provide a steel comprising 0.001 to 0.15%, preferably 0.005 to 0.05% of carbon, 0.01 to 1.0%, preferably 0.01 to 0.1% of silicon, 0.01 to 1.0%, preferably 0.1 to 0.3% of manganese, 0.01 to 0.5%, preferably 0.02 to 0.25% of copper, 0.003 to 0.03%, preferably 0.003 to 0.02% of sulfur, titanium in an amount enough to obtain a Ti/S ratio of more than 2, thus preferably 0.01 to 1.0% of titanium, optionally 0.1 to 1.0% of chromium and/or 0.001 to 0.05% of boron, with the balance being iron and unavoidable impurities.

conventionally, many steels having improved corrosion resistance in the air by addition of certain elements have been developed. For example, a steel containing copper, chromium, nickel and vanadium (Corten steel) and a steel containing copper, titanium and phosphorus are known. These steels, however, have been developed for the object to protect the metal inside the rust by the rust adhering on the matrix when exposed in the air, and not for the object to prevent or suppress the rust formation itself.

Therefore, the conventionally known art does not give any suggestion of the present invention which treats a completely different type of corrosion and corrosion mechanism.

Namely, the present invention has, as its object, to provide a steel sheet having excellent rust resistance under an indoor condition such as high moisture as in the summer season in Japan.

The basic elements composing the steel of the present invention, other than iron and carbon, are silicon, manganese copper, titanium and sulfur other than iron and carbon, and these elements are classified as the first group elements. The second group elements are chromium and/ or boron.

In the steel containing the first group elements, titanium in combination with copper contributes to improvements in the rust resistance, and remarkable rust resistance improvement is given when the ratio of T i/ S is 2 or more. The second group elements primarily tend to improve the rust resistance, and the rust resistance may be further improved by the presence of the first and second group elements.

Thus, the basic steel composition of the present invention comprises 0.001 to 0.15% of carbon, 0.01 to 1.0% of silicon, 0.01 to 1.0% of manganese, 0.01 to 0.5 copper, 0.003 to 0.03% of sulfur and titanium in an amount enough to have a Ti/S ratio of 2 or more, and a modified steel composition of the present invention comprises one or more of the second group elements in an amount of 0.1 to 1.0% for chromium and 0.001 to 0.05% for boron.

Limitation of each of the elements in the present invention is decided in view of the corrosion resistance, mechanical properties and steel-making problems.

The carbon content is necessary to give the desired strength, but more than 0.15 of carbon lowers the elongation of the steel and deteriorates the rust resistance. While less than 0.001% of carbon lowers the strength and requires a longer refining time, thus causing adverse effects on economics and productivity.

Silicon and manganese are necessary as deoxidizing elements in the steel making in an amount of not less than 0.01%, but excessive addition of these elements causes embrittlement of the steel and excessive strength. Thus the upper limits of these elements are limited to 1.0%.

Copper, when present with other elements, is effective to improve the rust resistance, but its effect is recognized when contained less than 0.01%, While more than 0.50% of copper causes embrittlement of the steel and deteriorates the workability.

It is desirable to reduce the sulfur content as low as possible, but from the economical limitations of the steel making, the sulfur content is limited to a range of 0.005- 003%. The titanium content is defined so that the ratio of Ti/S is not less than 2.

In case titanium is added to the steel, it is necessary to control the amount of titanium so that the ratio of titanium to sulfur in the steel can be not less than 2.

The reason for limitation of titanium is as follows: if the other sulfide former elements than titanium are not contained in the steel, the amount of titanium necessary for fixing all the sulfur in the steel is stoichiometrically 48/32 times more than the amount of sulfur. Since the commercial steel contains, however, manganese which is a typical sulfide former element. The above-mentioned amount of titanium are not enough for fixing all the sulfur in the steel as titanium sulfide and a part of the sulfide in the steel remains as manganese sulfide which causes rust formation.

Therefore, it is necessary for this purpose to increase the amount of titanium, so that the ratio of titanium to sulfur in the steel can be fixed at least more than the value of 2.0.

The reason for limiting chromium as an optional element to 0.1 to 1.0% is that when it is contained in an amount of 0.1% or more its effect on promoting the rust resistance is remarkable, but more than 1.0% of chromium brings only 'high production cost.

Boron is limited to 0.001 to 0.05% for the reason that less than 0.001% of boron show no substantial effect and more than 0.05% of boron causes embrittlement of the steel.

n the whole surface were placed in a moisture box maintained at 65 C, and their rust formation was determined.

(B) in the Table means that similar test pieces were exposed indoor for three months and their surface condition was observed. From the results of (A) and (B) of Table 1 it is very clear that the present inventive steel is much more excellent than the conventional steels in respect of rust resistance.

The above steel compositions may be produced by TABLE 1 Indoor Moisture testing exposure Hot Gold Gold Chemleal composition (percent) rolled, rolled, rolled, annealed annealed annealed C Si Mn T1 S Cu Cr B sheet sheet sheet Conventional steels What is claimed is: 1. A steel having excellent rust resistance consisting melting in an ordinary steel making furnace, such as an electric furnace and a converter in combination use of vacuum degassing, if necessary, then making slabs by continuous casting or ordinary ingot making method, hot rolling, cold rolling and annealing. In some cases, the steel sheet as hot rolled, or as hot rolled and normarized or annealed, or steel sheet as cold rolled may be used for the final product.

An example of the present invention will be set forth under.

Table 1 shows results of rust formation tests made on the present invention steels and convention steels both obtained by melting in a converter, ingot-making, hot rolling and annealing, and on the inventive steels and conventional steels both obtained by hot rolling, and then cold rolling and annealing.

(A) in Table 1 means that test pieces of 3 mm. thickness, 30 mm. width and 50 mm. length and butf polished essentially of 0.001 to 0.15% of carbon, 0.01 to 1.0% of silicon, 0.01 to 1.0% of manganese, 0.01 to 0.5% of copper, 0.003% of sulfur and titanium in an amount enough to attain a Ti/S ratio of 2 or more, with the balance benig iron and unavoidable impurities.

References Cited UNITED STATES PATENTS 2,474,766 6/1949 Waggoner 125 3,271,138 9/1966 Ohtakc 75-125 2,542,220 2/1951 Urban 75125 2,280,796 4/1942 Comstock 75-125 HYLAND BIZOT, Primary Examiner U.S. Cl. X.R. 75123 M 

